1. The Field of the Invention
The present invention concerns a system for controlling the operation of a bark-fired boiler. The system controls the position of the ash line and prevents craters in the bark.
2. State of the Art
In paper mills waste wood such as bark and wood chips is often burned, and the resulting heat is used to produce steam which in turn is used in the production of paper. Such boilers often have sloping grates upon which the wood is supported while it is burned. The bark is introduced onto the high end of the grate so that it travels downward while it is being burned. Air is introduced below the sloping grates so that the air travels upwardly through the bark thereby assisting combustion.
As the bark travels down the sloping grate the bark is often not uniformly disbursed across the grate. Where the bark is thinner air passes through the bark faster than where the layer of bark is thicker. This excess air enhances burning which in turn reduces the thickness of the layer of bark at the particular location thereby permitting additional increased air to flow through the bark. The result of this is that craters or holes can be formed in certain areas of the bark on the sloping grate while bark in other locations is only partially burned.
The formation of such craters or holes is undesirable since large quantities of air can flow through the craters and thereafter up through the furnace carrying large quantities of ash out of the boiler and through the stack. Also, these large quantities of air carry heat up the stack which is also undesirable. Presently operators of boilers watch for craters through ports in the boiler. If a crater is located, the operator alters the operation of the system to attempt to elliminate the craters.
Another parameter of interest in the operation of a bark-fired boiler is the location of the ash line. The ash line is essentially a line or zone of transition between burning wood and wood which has been burned to ash. It is normally preferable to operate the system with the ash line near the lower part of the grate so that the rate of through-put of bark is maximized. According to present practice the location of the ash line is observed by the operator of the boiler and manually controlled.